1. Field of the Invention
The present invention relates generally to an apparatus and method for transmitting and receiving data in a CDMA mobile communication system, and in particular, to a data transmission/reception apparatus and method for adjusting the reliability of data bits before transmission and weight-combining received data bits according to the reliability of received data bits to improve system performance.
2. Description of the Related Art
In a communication system, it is impossible to receive transmission signals without any distortion and noises mixed therein. In particular, a mobile communication system transmitting and receiving signals via a wireless network is more susceptible to the distortion and noises, compared with a wired communication system.
For this reason, there have been proposed various methods of reducing the influence of the distortion and noises on the mobile communication system. For example, in order to reduce a bit error rate from 10−2 to 10−3 in an AWGN (Additive White Gaussian Noise) environment using the typical modulation technique and coding technique, a low signal-to-noise ratio (SNR) of about 1 dB to 2 dB is required. On the other hand, in order to obtain the same results in a multipath fading environment, it is necessary to increase the signal-to-noise ratio to about 10 dB. However, a method of increasing transmission power in order to increase the signal-to-noise ratio for a reduction in the bit error rate may undesirably decrease the entire system performance. Therefore, a technique for effectively reducing or removing the influence of fading, i.e., the influence of distortion or noises, without additional power or a loss of bandwidth in both a UE (User Equipment) and a Node B is very important to the mobile communication system. One of the effective technique for this is a channel interleaving technology combined with an error control coding technique.
The channel interleaving technology interleaves transmission data bits before transmission to disperse the transmission data from a portion of a signal where data bits may be possibly damaged to different places instead of concentrating the portion on one place. That is, the interleaving technology prevents a burst error by allowing adjacent bits to be randomly affected by fading.
Meanwhile, codes used for the error control coding technique are divided into a memoryless code and a memory code. The memoryless code includes a linear block code, while the memory code includes a convolutional code and a turbo code. Further, a device for performing coding by the error control coding technique is called a “channel coder”.
In particular, a future mobile communication system requires reliable transmission of high-speed multimedia data, so it needs a more powerful channel coding technique. A channel coding technique using the turbo code shows performance nearest to the Shannon limit with respect to the bit error rate (BER) even in the low signal-to-error ratio. An output of a channel coder using the turbo code can be divided into systematic bits and parity bits. Here, the “systematic bits” refer to actual information data bits to be transmitted, while the “parity bits” refer to bits added to help a receiver correct a possible transmission error. However, even the error control coded signals cannot overcome a possible burst error occurring in the systematic bits or the parity bits.
Of course, the systematic bits and the parity bits output from the channel coder have different priorities. In other words, in the case where errors occur in transmission data at a given rate, it is possible to perform more correct decoding when the errors occur in the parity bits, compared with when the errors occur in the systematic bits. The reason is because, as stated above, the systematic bits are the actual information data bits, while the parity bits are the bits added to assist the receiver to correct transmission errors in decoding process.
The SMP (Symbol Mapping method based on Priority) technique can increases system performance by decreasing the probability that errors will occur in the systematic bits having higher priority than the parity bits. The SMP technique uses two interleavers at an output stage of the channel coder to map the systematic bits and the parity bits to the bits having different reliabilities. That is, an existing transmitter not employing the SMP technique interleaves the systematic bits and the parity bits together without discrimination before transmission. However, a transmitter employing the SMP technique separately interleaves the systematic bits and the parity bits, and then maps during modulation the bits having higher priority to a bit position having higher reliability and the bits having lower priority to a bit position having lower reliability. As a result, when transmitted by the SMP technique, the bits having higher priority have a lower bit error rate than the bits having lower priority, contributing to an increase in an error correcting capability by the receiver during decoding process.
FIG. 1 illustrates a signal constellation diagram for 16QAM (16-ary Quadrature Amplitude Modulation) modulation, and FIG. 2 illustrates a signal constellation diagram for 64QAM (64-ary Quadrature Amplitude Modulation) modulation.
Referring to FIG. 1, 16QAM-modulated symbols are each comprised of 4 bits, and have a reliability pattern [H,H,L,L], where H represents a bit position having higher reliability and L represents a bit position having lower reliability. That is, the leading two bits have higher reliability and the following two bits have lower reliability.
Referring to FIG. 2, 64QAM-modulated symbols are each comprised of 6 bits, and have a reliability pattern [H,H,M,M,L,L], where H represents a bit position having higher reliability, M represents a bit position having medium reliability and L represents a bit position having lower reliability.
Therefore, it is possible to map the interleaved systematic bits and parity bits to the bit positions with higher reliability, the bit positions with medium reliability and the bit positions with lower reliability according to systematic bits and priority bits priorities. Based on this, the SMP technique maps the systematic bits to the bit positions with higher reliability, resulting in an improvement in frame error rate (FER) during transmission. Here, as to the reliability, in a process of modulating one symbol by the transmitter, the symbol expressing two bits in a macro region like the left/right quadrants or upper/lower quadrants on the X/Y-axis of FIGS. 1 and 2 is said to have “higher reliability”, and the symbol expressing two bits in a micro region is said to have “lower reliability”.
FIG. 3 illustrates a structure of a transmitter constituting an HSDPA (High-Speed Downlink Packet Access) wireless communication system employing the SMP technique.
Referring to FIG. 3, tail bits are added to transmission data in the tail bit generator 111. transmission data is separated into systematic bits and parity bits by a channel coder 112, and then subjected to rate matching through a rate matcher 114. The rate-matched systematic bits and parity bits are provided to a distributor 116. The distributor 116 distributes the systematic bits with higher priority to a first interleaver 118 and the parity bits with lower priority to a second interleaver 120. The systematic bits interleaved by the first interleaver 118 and the parity bits interleaved by the second interleaver 120 are output in series through a parallel-to-serial converter 122, and then provided to a modulator 124. The modulator 124 maps before transmission to the receiver the systematic bits to the bit positions having higher reliability and the parity bits to the symbol positions having lower reliability. The transmitter of FIG. 3 is under the control of controller 126.
As described above, the SMP technique maps the systematic bits having higher priority to the bit positions with higher reliability, thus contributing to an improvement in FER during high-speed downlink packet transmission.
Meanwhile, since the future mobile communication system needs reliable transmission of high-quality, high-efficiency, high-capacity and high-speed multimedia data, it is necessary to link the SMP technique to the high-speed downlink packet transmission. However, when the SMP technique is employed in a system using a H-ARQ (Hybrid Automatic Repeat Request) technique, it is assumed that the bits having higher priority are mapped to the bit positions having higher reliability even at retransmission. In this case, the effect of a coding gain may be reduced, because decoding performance of a turbo decoder is improved when its input bits have a homogeneous LLR (Log Likelihood Ratio). The present invention provides a technique capable of obtaining both the coding effect and a diversity gain, and also capable of using the characteristic of the turbo decoder, in a system using the H-ARQ technique.